During the last ten years we have produced several unique strains of congenic mice. Because these have found cogent uses in several fields, notably immunology and its several sub-divisions, the genetics of cellular differentiation, and leukemia, we have had to expend much time, money and effort in supplying these mice to other research groups within and outside the USA. The demand for other such congenic strains which we are now deriving will be equally considerable, as their value in approaching questions of central relevance to molecular biology is becoming appreciated. It has been very costly for us to meet these obligations to outside investigators, and it will continue to be so. As there is no feasible way of recovering these costs from the outside investigators themselves, we ask for support of our specialized Mouse Breeding Unit on the grounds that it is, and will continue to be, a unique General Resource, freely available to investigators of many research disciplines and interests.